c++ - 神秘的链接器错误

Question

我编写了一个使用一些类和 Protocol Buffer 的简单程序。这些类是为了使计算机之间的连接和发送消息更容易。编译成功。但是，链接器说:

server.o: In function `main':

server.cpp:(.text+0x24): undefinedreference to`dataExchange::Server::Server(unsignedint)'

server.cpp:(.text+0x39):undefined reference to`dataExchange::Server::accept()'

server.cpp:(.text+0x51): undefinedreference to`dataExchange::Connection::receive()'collect2: ld returned 1 exit status

代码:服务器.cpp:

#include <iostream>

#include "connection.h"
#include "protocol.pb.h"

using namespace std;
using namespace msg;
using namespace dataExchange;

int main() {
  Server server(1234);
  while(1) {
    Connection con = server.accept();
    Annoucement ann = con.receive();
    cout << ann.typ() << endl;
  }
}

连接.h:

namespace dataExchange {

  class SocketMaintenance {
  protected:
    sockaddr_in addr;
    int s;
  public:
    SocketMaintenance(const unsigned int) throw();
    SocketMaintenance(const sockaddr_in, const int) throw();
    SocketMaintenance(const SocketMaintenance&) throw();
    ~SocketMaintenance();
    void write(const char*, const int);
  };

  class Connection {
    FileOutputStream* raw_output;
    FileInputStream* raw_input;
    char buffer[1024];
  public:
    Connection(const char*, const unsigned int);// throw(WrongAddress);
    Connection(const SocketMaintenance&);//throw(ConnectFailed);
    void send(const Message&) throw();
    Annoucement receive() throw();
  };

  class Server {
  public:
    Server(const unsigned int);// throw(BindFailed,ListenFailed);
    Connection accept() throw();
  };

}

链接器失败的原因是什么？

---

构建过程:

g++ server.cpp -c; g++ 协议(protocol).pb.cc -c； g++连接.cpp-c；g++ server.o protocol.pb.o connection.o -lprotobuf

连接.cpp

using namespace std;
using namespace msg;
using namespace google::protobuf;
using namespace google::protobuf::io;

namespace dataExchange {

  class SocketMaintenance {
  protected:
    sockaddr_in addr;
    int s;
  public:
    SocketMaintenance(const unsigned int port) {
      memset(&addr, 0, sizeof(addr));
      addr.sin_family = AF_INET;
      addr.sin_port = htons(port);
      s = socket(AF_INET, SOCK_STREAM, 0);
    }
    SocketMaintenance(const sockaddr_in Addr, const int Socket) {
      addr = Addr;
      s = Socket;
    }
    SocketMaintenance(const SocketMaintenance& source) {
      addr = source.addr;
      s = source.s;
    }
    ~SocketMaintenance() {
      close(s);
    }
    void write(const char* buffer, const int n) {
      ::write(s, buffer, n);
    }
  };

  class Connection : public SocketMaintenance {
    FileOutputStream* raw_output;
    FileInputStream* raw_input;
    char buffer[1024];
  public:
    Connection(const char* IP, const unsigned int port) : SocketMaintenance(port) {
      if(inet_pton(AF_INET, IP, &addr.sin_addr)<=0) {
    throw WrongAddress();
      }
      if(connect(s, (sockaddr*)&addr, sizeof(addr))<0) {
    throw ConnectFailed();
      }
      raw_output = new FileOutputStream(s);
      raw_input = new FileInputStream(s);
    }
    Connection(const SocketMaintenance& source) : SocketMaintenance(source) {}
    ~Connection() {
      delete raw_output;
    }
    void send(const Message& msg) throw(EmptyMessage)  {
      CodedOutputStream* coded_output = new CodedOutputStream(raw_output);
      int n = msg.ByteSize();
      if(n<=0) throw EmptyMessage();
      coded_output->WriteVarint32(n);
      delete coded_output;
      raw_output->Flush();
      msg.SerializeToArray(buffer, n);
      SocketMaintenance::write(buffer, n);
    }
    Annoucement receive() throw() {
      CodedInputStream* coded_input = new CodedInputStream(raw_input);
      google::protobuf::uint32 n;
      coded_input->ReadVarint32(&n);
      char *b;
      int m;
      coded_input->GetDirectBufferPointer((const void**)&b, &m);
      Annoucement ann;
      ann.ParseFromArray(b, n);
      return ann;
    }
    
  };

  class Server : public SocketMaintenance {
  public:
    Server(const unsigned int port) : SocketMaintenance(port) {
      addr.sin_addr.s_addr = htonl(INADDR_ANY);
      if(bind(s, (struct sockaddr *)&addr, sizeof(addr))<0) {
    throw BindFailed();
      }
      if(listen(s, 5)<0) {
    throw ListenFailed();
      }
    }
    Connection accept() throw() {
      sockaddr_in client;
      socklen_t client_len;
      int client_socket = ::accept(s, (sockaddr*)&client, &client_len);
      return Connection(SocketMaintenance(client, client_socket));
    }
  };

}

Answer 1

很可能忘记编译和/或链接 “connection.cpp”。我至少希望你有这样一个文件，它实现了所有的功能？

---

我认为您对 .h 和 .cpp 的拆分方式存在一些根本性的误解。在 .cpp 中，您不需要简单地重新声明类，这次是使用在其中定义的函数。你需要这样做:

// connection.cpp
namespace dataExchange{
// define the constructor of Server
Server::Server(const unsigned int port)
  : SocketMaintenance(port)
{
  // ....
}

// define the accept function of Server
Connection accept() throw(){
  // ...
}
// ...
}

但这只是问题的一部分，看来你需要得到a good C++ book .例如，您需要在声明类的 header 中进行任何继承。